1. Field of the Invention
The present invention relates to a nozzle, especially a nozzle for use in a geothermal steam turbine, and more particularly to a nozzle for use in a geothermal steam turbine which is adapted to prevent precipitation and adhesion of scale onto a nozzle surface, and a method for preventing the adhesion of scale onto the nozzle.
2. Description of the Prior Art
The purity of steam in a geothermal power plant is a factor exerting a very large influence upon the reliability of the power plant. More particularly, SiO.sub.2, Fe, Na, Cl and the like contained in steam have the tendency of precipitating and adhering as scale 4, mainly to a rear surface 2 and a concave front surface 3, of a first stage nozzle 1, as shown in FIG. 3. Adhesion of scale onto the first stage nozzle does not only bring about a lowering of the output power, caused by a reduction of the nozzle area, but also could possibly result in a breakdown or accident of the rotor (moving blades or a disc) due to contact between the scale and the rotor. Accordingly, the adhesion of scale onto a nozzle is a factor largely governing the overhaul interval of a geothermal power plant. From the above-mentioned reasons, establishment of a technique for preventing the adhesion of scale to a nozzle is an essentially necessary problem in view of both the aspects of insuring reliability of the geothermal power plant and improving the availability factor of the same.
On the other hand, one currently practiced technique for preventing adhesion of scale is a method illustrated in FIG. 4, which has been known. More particularly, this method is a method consisting of the steps of extracting a part of steam 13 fed to a geothermal steam turbine 12 through an inlet steam pipe 11 and condensing the part in a condenser 14, then injecting the thus condensed water 15 into the steam 13 flowing through the inlet steam pipe 11 by pressurizing it with a pump 16, and thereby water-washing out the scale adhering to the nozzle within the turbine 12. It is to be noted that reference numerals 17 and 18 designate valves.
As described above, the method known in the prior art is a method of water-washing scale adhered to a nozzle by injecting condensed water 15 prepared from geothermal steam into the inlet steam pipe 11 of the geothermal steam turbine 12. This method is generally called the "water-washing method". However, according to practical results in a practically used machine, there are some plants in which this method is not always effectively practiced.
Considering the reasons of the unavailability of a method for prevention precipitation in the prior art, it appears that attention was paid solely to removal of the precipitated and adhered scale, and a fundamental countermeasure for removing the cause was not taken. Therefore, the inventors of the present invention at first investigated the mechanism of "adhesion of scale". The results of an examination of a composition of moisture mixed in the steam of the investigated geothermal power plant were:
______________________________________ Na 0.2 ppm as NaCl . . . 0.55 ppm Cl 0.35 ppm SiO.sub.2 0.066 ppm ______________________________________
It was further investigated, with respect to a rear surface and a concave front surface of the nozzle, how these components adhere to the nozzle surfaces. The results were as follows: EQU &lt;Rear Surface of Nozzle&gt;
As shown in FIG. 5(A), at the portion downstream of Point .circle. , which is about 0.55 (width) apart from the front edge of the nozzle, the metal surface temperature is higher than the steam temperature at the blade surface. Consequently, drain would reevaporate from the metal (blade) surface, and NaCl and SiO.sub.2 would condense and would be precipitated as scale.
However, it is to be noted that, whereas a percentage content of NaCl is 0.55 ppm, that of SiO.sub.2 is as small as 0.066 ppm, and moreover, since SiO.sub.2 is liable to transfer to the vapor phase in view of the distribution rate which is characteristic to SiO.sub.2 and is liable to disperse into the steam flow, the principal component of the scale is NaCl.
From the above-mentioned reasons, the following conclusion is obtained:
(1) Scale would precipitate on the downstream side of the position about 0.55 (width) apart from the front edge. PA1 (2) A principal component of the scale is NaCl. &lt;Front Surface of Nozzle&gt; PA1 (1) Scale would precipitate in the proximity of the rear edge. PA1 (2) A principal component of the scale is SiO.sub.2.
As shown in FIG. 5(B), since a metal (blade) surface temperature is lower than a steam temperature at the surface of the blade in nearly all the range, condensation of steam into drain would proceed on the surface of the blade, but reevaporation of the drain would not occur. Since the solubility of NaCl is large, if drain is present, NaCl would not precipitate as scale. Accordingly, on the front (concave) surface of the nozzle where, reevaporation of drain would not occur, NaCl could never precipitate as scale. On the other hand, with respect to SiO.sub.2, at the location behind the Point .circle. (the position about 0.95 (width) apart from the front edge) where condensation of drain commences gradually, it precipitates as scale by the amount exceeding its solubility.
From the above-mentioned reasons, the following conclusion is obtained:
Further, with respect to Fe, it precipitates at the location where drain condensates.